1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a reflective liquid crystal display device using a cholesteric liquid crystal color filter layer.
2. Discussion of the Related Art
A liquid crystal display device is thin, portable, low weight and low power consumption. The liquid crystal display device is a technology-intensive and value-added product and the liquid crystal display device is the next generation display device.
Among the various types of liquid crystal display devices commonly used, active matrix liquid crystal display (AM-LCD) devices, in which thin film transistors (TFTs) and pixel electrodes connected to the TFTs are disposed in matrix, have been developed because of their high resolution and superior display of moving images.
In general, the process of forming the liquid crystal display device includes forming switching devices and pixel electrodes on an array substrate, forming a color filter substrate with a color filter layer and a common electrode, and a liquid crystal cell process where a liquid crystal is interposed between the array substrate and the color filter substrate. Further, because the liquid crystal display device is a light-receiving type display device, a backlight device is required to supply light and display images. However, only about 7% of the light generated from the backlight device can pass through the liquid crystal cell. For this reason, the backlight device requires a high, initial brightness, and thus electric power consumption by the backlight device increases. A relatively heavy battery is needed to supply a sufficient power to the backlight of such a device.
To solve these problems, a reflective liquid crystal display device has been researched and developed. Because the reflective liquid crystal display device operates using ambient light other than an internal light source such as a backlight device, battery life can be increased resulting in longer use times. Namely, only the drive circuitry that drives the liquid crystal uses the battery power in the reflective liquid crystal display device.
For the reflective liquid crystal display device, a reflector and/or and a reflective electrode is arranged in a pixel region where the transparent electrode is formed in a transmissive liquid crystal display device. In other words, the reflective liquid crystal display device is driven using the light reflected from the reflective electrode or/and the reflector. However, the reflective liquid crystal display device is low in brightness due to the fact that the reflective liquid crystal display device uses the ambient light and the brightness depends on this ambient light from surroundings. One of the reasons for the low brightness is that the ambient light passes through the color filter twice. Due to the reflection on the reflector, the incident light from the outside passes the color filter and then is reflected from the reflector. Then, it is directed toward the color filter again. Therefore, most of the light is absorbed by the color filter, thereby decreasing the brightness.
In order to overcome above-mentioned problem, it is essential to raise the transmittance of the color filter. Further, to get the excellent transmittance, the color filter ought to have low color purity. However, there is a limitation of lowering the color purity.
Accordingly, to improve the operating characteristics (such as brightness) of the reflective liquid crystal display device, a cholesteric liquid crystal (CLC) has been studied and developed, which selectively transmits or reflects the light with a specific color. If the CLC color filter is used in the reflective LCD device, it is possible to omit the reflector from the reflective LCD device, thereby simplifying the manufacturing processes. Furthermore, it has the advantage of increased color purity and contrast ratio.
The CLC has a helical shape and the pitch of the CLC is controllable. Therefore, the CLC color filter can selectively transmit or/and reflect the light. In other words, as is well known, all objects have their intrinsic wavelength, and the color that an observer recognizes is the wavelength of the light reflected from or transmitted through the object. The wavelength (λ) of the reflected light can be represented by a following functional formula of pitch and average refractive index of CLC; λ=n(avg)·pitch where n(avg) is the average index of refraction. For example, when the average refractive index of CLC is 1.5 and the pitch is 430 nm, the wavelength of the reflected light is 650 nm and the reflective light becomes red. In this manner, the green color and the blue color also can be obtained by adjusting the pitch of the CLC.
In other words, the wavelength range of visible light is about 400 nm to 700 nm. The visible light region can be broadly divided into red, green, and blue regions. The wavelength of the red visible light region is about 660 nm, that of green is about 530 nm, and that of blue is about 470 nm. Due to the pitch of the cholesteric liquid crystal, the CLC color filter can selectively transmit or reflect the light having the intrinsic wavelength of the color corresponding to each pixel thereby clearly displaying the colors of red (R), green (G) and blue (B) with a high purity. In order to implement a precise color, a plurality of the CLC color filters can be arranged, therefore the CLC color filter can display the full color more clearly than the color filter conventionally used. The cholesteric liquid crystal (CLC) color filter will be referred to as CCF herein after.
FIG. 1 is a schematic cross-sectional view illustrating a display area of a reflective liquid crystal display (LCD) device having a CCF (cholesteric liquid crystal color filter) layer according to a related art.
As shown, a reflective LCD device includes lower and upper substrates 10 and 30 and an interposed liquid crystal layer 50 therebetween. The lower and upper substrates 10 and 30 include transparent substrates 1, respectively, such as glass.
On the surface facing the upper substrate 30, the lower substrate 10 includes a light-absorbing layer 12. An alignment layer 14 is disposed on the light-absorbing layer 12. A CCF (cholesteric liquid crystal color filter) layer 16 including red (R), green (G) and blue (B) CLC color films 16a, 16b and 16c in sub-pixels are disposed on the alignment layer 14. A common electrode 18 is disposed on the entire CCF layer 16. The light-absorbing layer 12 selectively absorbs some portions of light incident from the CCF layer 16, and the alignment layer 14 aligns and orients the cholesteric liquid crystals formed thereon.
Still referring to FIG. 1, on the surface facing the lower substrate 10, the upper substrate 30 includes a switching device, such as a thin film transistor T, and a pixel electrode 32 in each sub-pixel. The pixel electrodes 32 apply voltage to the liquid crystal layer 50 with the common electrode 18. On the other surface, the upper substrate 30 includes a retardation layer 34 and a polarizer 36 in series. The retardation layer 34 is a quarter wave plate (QWP) that has a phase difference of λ/4 (lambda/4), and the polarizer 36 is a linearly polarizing plate that only transmits portions of light parallel with its polarizing axis.
In the reflective LCD device shown in FIG. 1, the CCF layer 16 produces colors and also acts as a reflector reflecting light. Therefore, the brightness of the reflective LCD device of FIG. 1 fully depends on the reflecting characteristic of CCF layer 16.
FIG. 2 is a graph illustrating spectrums of light reflected by red, green and blue CLC color films of FIG. 1.
In FIG. 2, the CCF type reflective LCD device has peak wavelengths Ia, Ib and Ic corresponding to the red, green and blue CLC color films, and the peak points Ia, Ib and Ic are 650 nm, 550 nm and 450 nm, respectively. The cholesteric liquid crystal material of the CLC color films has a birefringence of about 0.15, and thus the maximum width of each wavelength, especially the green wavelength, is 50 nm as shown in FIG. 1. This means that the reflectance of the reflective LCD device decreases.
As compared with the CCF type reflective LCD device, a light-absorbing type reflective or transmissive LCD device has a color filter that only transmits the portion of light matching with the color filter wavelength and absorbs the other portions of light. Therefore, the thickness of the light-absorbing type color filer is in inverse proportion to the reflectance and in proportion to the color purity. That is, the brightness and contrast ratio of the light-absorbing type reflective/transmissive LCD device is adjustable by way of controlling the thickness of the light-absorbing type color filter layer. However, because the CCF layer has its own pitch and that pitch is determined when the LCD device is designed, it is very difficult for the CCF type reflective LCD device to adjust and control the color purity and reflectance in the same way as the light-absorbing type reflective/transmissve LCD device does.
The red, blue and green colors of the CCF layer are controlled and achieved by the pitch and birefringence of the cholesteric liquid crystal of each sub-pixel. As the birefringence becomes larger, the reflected wavelength band also becomes wider. By adjusting the reflectance in the reflected wavelength band, the desired color purity and brightness can be achieved. However, it is very difficult to create and develop the cholesteric liquid crystal material that has a large birefringence because the manufacturing cost increases as the birefringence increases.